1. Field of the Invention
The present invention relates generally to domestic water heating systems used in residential and commercial applications and, more specifically, to external tankless domestic hot water coils and to systems of controlling their operation.
2. Description of the Prior Art
At the present state of the art in domestic hot water heating systems two methodologies are primarily employed. The first requires the use of a separate domestic hot water heating system with separate heat source, typically oil, gas or electricity. This type of installation requires a large outlay of time and economic resources since the systems are expensive to purchase, expensive to install and require a great deal of storage space. The second methodology places a "tankless coil" into or adjacent to the source of heat used for space heating. While relatively simple to install, this technique provides limited volumes of domestic hot water since water to be heated passes once through the coil. Even very large coils yield limited hot water whose temperature falls as demand increases.
Numerous efforts have been made to find alternative techniques for heating domestic hot water. Joseph Roy et. al. in U.S. Pat. No. 4,037,779 provided a heating system having high-low temperature limit controlled auxiliary boiler; however, this system has a separate boiler with all the inherent disadvantages of the first methodology already described.
Charles Staats in U.S. Pat. No. 4,416,222 provided hot water heater circuitry which advanced the state of the art by providing a system whereby a single opening or port may be used for both inletting and outletting water from a typical hydronic boiler to an auxiliary hot water heater. However; this improvement does not address the problem of controlling flow through the auxiliary heater and does not address the means by which the auxiliary water heater operates; only the porting techniques.
Chetwood Smith in U.S. Pat. No. 1,591,647 provides an indirect domestic water heating system which uses an external heat exchanger to supply domestic hot water but controls the flow, and thereby the water temperature by a manually operated valve which provides a very limited ability to control water temperature.